The two primary ways in which the functional redundancy of cytokines is manifested are the binding of multiple cytokines by a single cytokine receptor or the sharing of a cytokine receptor subunit by multiple cytokine receptors. The functional redundancy of cytokines in vivo is illustrated by the characterization of the phenotypes of mice or humans that have mutations in a cytokine or a cytokine receptor subunit. The sharing of gamma-c by 5 cytokine receptors (IL-2R, IL-4R, IL-7R, IL-9R and IL-15R) raises several important questions concerning the molecular basis of the ligand:gamma-c interaction. These include: How does a single receptor subunit such as gamma-c contribute to the binding of 5 different cytokines? Is gamma-c involved in receptor:receptor interactions to stabilize ligand binding? What limits the cell surface levels of gamma-c? One major goal of this proposal is to address these issues to perform structure/function analysis of gamma-c by site-directed and deletion mutagenesis. Signaling through the IL-2R, IL-4R and IL-7R leads to growth and differentiation of mature T lymphocytes into effector cells. There appear to be at least two ligands that bind to each of these receptors. Some of these cytokines are also critical for T and B cell development. It has been difficult, therefore, to ascertain the roles of IL-2R, IL-4R and IL-7R in regulation of a T-cell response by analysis of "knockout" mice that lack a cytokine or an individual receptor subunit. Therefore, the second major goal of this proposal is to selectively inhibit the function of IL-2R, IL-4 and IL-7R in mature T lymphocytes by the production of transgenic mice that express dominant-negative receptor subunits and assess the resulting affect on T-cell function. The specific aims are: 1) To define subregions in the extracytoplasmic region of gamma-c that participate in ligand binding or receptor:receptor interactions. 2) To determine the subregion within the cytoplasmic tail of gamma-c that regulates cell surface levels and investigate whether this region also controls ligand:receptor internalization. 3) To distinguish the relative roles of gamma-c, IL-2R beta, IL-4R alpha and IL-7R alpha in the function of mature T lymphocytes in vivo.